Image-forming MR methods which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects and do not require ionizing radiation and they are usually not invasive.
According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicularly to the z-axis.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collection a number of samples. A sample of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
Coronary magnetic resonance angiography (CMRA) has been shown to allow accessing the status of the coronary tree or the patency of corresponding bypass grafts. Whole heart approaches, benefiting from parallel reception have also been introduced to image the entire coronary tree in a single acquisition. To improve the visibility of the coronaries in their epicardial bed, fat suppression is usually employed in CMRA.
For fat suppression basically two types of approaches are used. Chemical shift selective pre-saturation uses the principle that the longitudinal magnetization of fat is selectively excited and subsequently dephased and thus suppressed. Spatial excitation is an alternative, exciting only the resonance of interest, e.g. water, ignoring the fat. Also for other cardiac scan protocols fat is often suppressed to increase the overall image contrast.
However, the suppressed fat signal also contains helpful diagnostic information for a number of cardiac diseases. For example in myocardial infarction or in the presence of suspicious cardiac masses, the intra-myocardial fat represents an important diagnostic indicator. It was even reported that fibro-fatty infiltration of the myocardium is associated with sudden death. Thus, the fat signal and its distribution may have higher prognostic value.
To address coronary artery disease and potential myocardial fat infiltration, today basically two individual scans have to be made. However, the performance of two scans has the disadvantage that it takes a rather long period of time in order to perform the scans.
An alternative way to obtain both information on water and fat contributions to the MR signal is chemical shift encoding, in which an additional dimension to the measurement data is defined and encoded, by performing a couple of additional image data acquisitions with slightly different echo times. For water-fat separation these types of experiments are often called Dixon-type of measurements. By means of Dixon imaging or Dixon water/fat imaging, a water-fat separation can be obtained by calculating contributions of water and fat from two or more corresponding echoes, acquired at different echo times. Dixon imaging usually relies on the acquisition of at least two echoes to separate water and fat signals. In general these kind of separations are possible because there is a known precessional frequency difference of hydrogen in fat and water. In its simplest form, water and fat images are generated by either addition or subtraction of the ‘in phase’ and ‘out of phase’ datasets, but this approach is rather sensitive to main field inhomogeneities. More advanced water/fat Dixon separation methods allow also an estimation of the main field inhomogeneity map. In that respect these chemical shift encoding techniques can have serious advantages over the above mentioned chemical shift selective approaches which might have image quality problems in case of large main field inhomogeneities.
Consequently, chemical shift-based water-fat separation methods require the acquisition of two or more images at different echo times, which is also prolonging the total scan time. The acquisition of several images (3D) could take too long to be performed which may cause inconsistencies between images at different echo times for example due to motion artefacts in coronary imaging. Consequently, Dixon-type of measurements have rarely been used in cardiac magnetic resonance.
For example Kellmann et al, Magnetic Resonance in Medicine 61, pages 215-221, 2009 discloses a multi-echo Dixon fat and water separation method for detecting fibro-fatty infiltration in the myocardium. However, only standard inversion recovery pulse sequence have been used to detect the fatty infiltration in chronic myocardial infarction which only provides MR image data at rather limited quality which again limits the visibility of the coronaries in their epicardial bed.
From the forgoing it is readily appreciated that there is a need for an improved MR imaging method. It is consequently an object of the invention to enable MR imaging in a fast manner by providing combined information on the coronary tree as well as on potential fibro-fatty infiltration of the myocardium. Further, from the forgoing it is readily appreciated that there is a need for an improved MR imaging system and an improved computer program product adapted to carry out the method according to the invention.